Based partly on this thread http://www.advrider.com/forums/showthread.php?t=500968
I think the valve on the fuel tank is a 2 way valve. It allows air in to deal with vacuum in the tank via the canister and allows pressure out also via the canister (tube that goes to the throttle body).
If some one wants to test that theory, take the hose off at the tank that goes to the canister and see if you can suck air in and blow it out. My guess is it only works in one direction, or at least easier to suck than blow.
I suspect the breather valve in Yeti's diagram is an on demand switch that controls when the fumes in the canister are drawn into the throttle body.
I know it takes pressure to over come the tank valve to allow fumes out. I have heard someplace it is 4.5lb.
Even if you remove the charcoal canister you will still get the build up of pressure in the tank until you trip the pressure release valve at the tank. That is how mine works. BTW if you do remove the canister do not cap the hose to the tank.